A correct uterine milieu is essential for the successful initiation of pregnancy during the pre-implantation period. Hormonally-induced changes in protein synthesis may alter the composition of uterine secretions on which the developing embryo depends for its nurture. In the rabbit, a uterine protein called "uteroglobin" has been implicated in the growth of the blastocyst, but its role has not yet been clearly defined. Since the protein is regulated by progesterone and estradiol, we propose to study the mechanism of action of progesterone on uteroglobin synthesis, by molecular hybridization of mRNA isolated after hormone treatment, using a complementary DNA probe. We will investigate also the involvement of progesterone receptor, by studies of chromatin transcription in vitro. The effects of estradiol on this mechanism will be investigated. Knowledge of the precise mechanisms by which ovarian steroids regulate the synthesis of specific proteins in uterine secretions will contribute to understanding of the interplay between the uterus and the blastocyst during development of the fertilized egg and implantation.